Presently, biologically compatible air-based pressure monitoring catheters are used in a number of medical applications to monitor pressure at various locations within a mammalian body. For example, air-based pressure monitoring catheters may be inserted into the skull of a patient thereby permitting the external monitoring of intra-cranial pressure.
Currently, a number of air-based pressure monitoring catheters have been developed. Generally, these air-based pressure monitoring catheters comprise a catheter having an air lumen formed therein which communicates with a bladder positioned at or near its distal end. In addition, the catheter includes a connector located at or near its proximal end, which may be connected to an external pressure transducer.
During use, the volume of the bladder attached to the catheter changes as pressure varies in accordance with Boyle's Law (P1V1=P2V2). As a result, the pressure of the gas within the catheter becomes equal to that of the environment surrounding the bladder. The media surrounding the bladder must be capable of movement to accommodate the variations in bladder volume as pressure changes.
The use of air-based pressure monitoring catheters in low or negatively pressurized environments has proven problematic. When the proximal connector is open to atmospheric pressure in the process of periodically replacing air lost by diffusion through the bladder, the external pressure extant in the body site monitored on a bladder will expel residual air from the bladder. If the pressure is low or negative, a significant amount of residual air may remain in the bladder. The amount of air injected is intended to be sufficient to keep the bladder in an active state for a period of 8 hours. If this volume is added to the residual air in a bladder that has not been completely collapsed by the environment around it, the sum of the residual air and injected air exceed the intrinsic volume of a fully shaped bladder. Should this happen, a positive pressure is established in the bladder. The bladder is now unable to read pressure below the internal pressure created.
Air management systems such as those seen in U.S. Pub. No. 2007/0208270 and U.S. Pat. Nos. 6,447,462 and 8,360,988, which are all herein incorporated by reference, allow a user to adjust the amount of air in a system. For example, these systems allow a user to vent the air passage of the catheter to the open environment, then charge the passage with an amount of air. In this respect, the pressure in the resulting charged passage can be monitored and a pressure within the patient (e.g., within the patient's skull) can be determined.
To function properly, the air in these catheters with air-based pressure sensors must be free to move within the lumen without obstruction. If a small amount of water should get in the lumen, air is constrained from moving freely, preventing an accurate pressure reading from being determined.